This invention relates to programmable attenuators, and more particularly, to a highly accurate programmable attenuator having particular advantages at high frequencies.
A programmable attenuator is a device which allows for the attenuation of a microwave or RF signal to be adjusted to a predetermined value within a particular setup. The accuracy of programmable attenuators is dependant upon how closely the actual attenuation is to the expected or selected value at a particular frequency or over a given frequency range. The accuracy of a programmable attenuator is measured by stating the number of decibels (dB) that the measured value deviates from the selected value. This deviation may, of course, vary from one frequency to another frequency. A programmable attenuator with a deviation greater than 0.5 dB is generally considered to have low accuracy, whereas deviations between 0.2 and 0.5 dB are considered to have medium accuracy and deviations of less than 0.2 dB are considered high accuracy. An ideal programmable attenuator would have a deviation of less than 0.1 dB but such attenuators have been generally impractical with current technology. However, this invention discloses a method for mass producing programmable attenuators with a deviation of 0.1 dB or less.
In general, a digital programmable attenuator is a device consisting of one or more attenuators connected to an equal number of switches. The switches (which may be either mechanical or electrically controlled) are wired in the circuit with the attenuators so that the microwave or RF signal is forced to travel either through an attenuator or directly across the switch, imparting little or no attenuation to the signal. The common way of configuring a programmable attenuator is by using six attenuators positioned in a binary sequence, so that each successive attenuator is twice the decibel value of the previous one. The binary sequence arrangement allows for the selection of multiple values of attenuation through the combination of particular attenuators chosen from the sequence. The accuracy of the entire programmable attenuator is largely dependent upon the accuracy of the individual attenuators making up the circuit. Accuracy is determined by adding the accuracy of each attenuator and obtaining a final figure. Accordingly, if each attenuator has an error of approximately 0.1 dB and there are six individual attenuators in the circuit, the total error for the circuit equals 0.6 dB.
Currently, it is the practice in trying to obtain high accuracy attenuators to select individual attenuators having margins of error above and below nominal so that the errors cancel out. Unfortunately, this is a time consuming, and therefore expensive task. In addition, the switches which also make up the attenuator circuit provide an inherent loss thereto. That is, each switch of a circuit may vary as to accuracy up to as much as 0.2 dB per switch. Therefore, even if highly accurate individual attenuators are obtained, the switches themselves making up the circuit taint the accuracy of the entire programmable attenuator. Unfortunately, there is no way of determining the loss through the switches until they are arranged in the circuit.
Because of the inherent losses in the switch, in designing the programmable attenuator, attenuation is always referenced to the zero decibel or "insertion loss" state. In this state, all of the individual attenuators are switched out of the circuit and the signal travels directly through the switches without any interruption. However, as mentioned, all switches used in programmable attenuators inherently have some loss which affects the signal traveling therethrough. The measure of this loss is commonly referred to as the insertion loss and attenuation is equal to the combined attenuator values minus the insertion loss. Designers strive to make the insertion loss as small as possible. However, it can be quite significant and somewhat unpredictable. Therefore, the attenuation of the circuit must be referenced to the loss associated with the switches. To acquire the desired attenuation, this loss must be accounted for by increasing the attenuation of the individual attenuators.
The conventional way of dealing with the variation and loss of the switches used for the circuit is to use manufacturing history to guess at the value of each attenuator and its switch. After guessing, the attenuators are fixed to the circuit and a final value for the programmable attenuator is determined. If this value is still greater than the desired attenuation, attenuators and switches are removed and replaced in the circuit until the desired value is obtained. This process, while marginally effective, is extremely time consuming and it is often impossible to find attenuators having the attenuation value that is required for precisely designing the circuit.
The prior art discloses several patents directed to compensating for impedance error within an attenuator circuit. U.S. Pat. No. 4,138,637 to Weinhart, for example, discloses a step attenuator including means for compensating for impedance errors in a circuit. While Weinhart does disclose the use of a shunt resistor, it is a fixed value which simply adds to the problems discussed above.
U.S. Pat. No. 4,952,893 to Cuddy discloses an attenuating circuit having an attenuation signal path and a reference path wherein the reference path is adapted to be attenuated by an amount related to the attenuation path for the purpose of improving accuracy. However, the adjustments and compensation to the circuit for desired attenuation are performed on the attenuation path and not the reference path seemingly as described above and unlike the instant invention.
The invention described herein however, discloses a method and apparatus for producing an ultrahigh accuracy programmable attenuator without having to repeatedly remove and replace components of the circuit. Therefore, considerable time and effort is saved which is also reflected in the cost of the device.